Field of the Disclosure
The present disclosure relates to vapor-deposited or sputtered nitride coatings used to extend the service lives of metal tooling, and more particularly to methods for stripping nitride release coatings from tools such as metal glass molds that have been partially oxidized by the adverse conditions of use.
Technical Background
Evaporated or sputtered nitride coatings, including, for example, TiN, TiAlN, CrN, TiAlCrN, TiAlSiN, AlN and the like, have been used to improve the wear resistance of metal tooling (“wear” coatings) or to improve the release characteristics of metal surfaces (“release” coatings). One particularly demanding application for such coatings is as a release coating for glass molds. Advanced glasses for technical applications exhibit softening points in the range of 800° C., and molding complex shapes from such glasses requires the use of refractory metal molds with release coatings exhibiting physical as well as chemical stability at such temperatures. Nitride coatings, such as TiAlN coatings, applied by PVD (physical vapor deposition) can provide high temperature oxidation resistance, good release characteristics from softened glass, and high corrosion resistance for enhancing metal mold life and maintaining molded glass surface quality.
Nevertheless hard evaporated or sputtered coatings, including TiAlN coatings, can degrade after prolonged thermal cycling in contact with hot glass. Cracking of the coatings and degraded glass release properties are indicators of coating deterioration. In order to preserve and extend the service lives of the expensive glass molds used for the shaping of these glasses, an efficient process for stripping the nitride coatings from the molds is required. Further, the stripping process employed must leave mold surfaces in a condition suitable for the re-application of a new release coating.
Among the methods having potential application for the stripping of nitride wear coatings are DC or RF plasma etching, chemical stripping utilizing highly alkaline aqueous solutions with or without additions of oxidizers such as permanganates or peroxides, and electrochemical stripping. Plasma etching is slow and expensive, and typically requires line-of-sight access to the coated surface. Chemical stripping is also relatively slow, generally requiring the use of hot corrosive solutions that involve significant safety issues and process energy requirements, with multiple solutions of differing composition and temperature being needed to achieve full coating removal without substrate surface damage.
In contrast to wear coatings for conventional tooling, evaporated or sputtered nitride coatings employed as release coatings for glass molding applications present a unique stripping challenge due to changes in coating characteristics caused by the environment of use. Repeated thermal cycling to high temperatures during the pressing of molten glass articles causes substantial changes in composition as well as morphology in glass mold release coatings. Among the changes observed are the development of oxide phases in surface regions of the coatings that produce partially oxidized surface layers, and the formation of intermetallic phases due to the migration of metal species from the metal mold surfaces into base portions of the release coatings.
These changes, together with changes in crystallinity in the bulk coating material, cause substantial variations in stripping behavior, whereby treatments effective for removing an as-applied nitride coating system are ineffective for the stripping of the same system after prolonged thermal cycling of the coatings.